The present invention relates to rotary electric motors, more particularly to a permanent magnet motor having a radial air gap of varying dimension between interfacing rotor permanent magnets and stator poles.
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient and flexible operating characteristics. In a vehicle drive environment, for example, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Such a vehicle motor drive should advantageously provide ready accessibility to the various structural components for replacement of parts at a minimum of inconvenience. The above-identified copending related U.S. applications describe formation of electromagnet core segments as isolated magnetically permeable structures configured in an annular ring. With such arrangements, flux can be concentrated to provide advantageous effects as compared with prior art embodiments.
As described in the above-identified Maslov et al. applications, isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects from interaction with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an isolated electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched. The lack of additional poles within the group avoids any such effects within a group. Further benefits are described from utilization of three dimensional aspects of motor structure, such as a structural configuration wherein axially aligned stator poles and axially aligned rotor magnets provide highly concentrated flux density distribution in the active air gap of the machine. Such configuration provides a greater number of poles with the same individual active air gap surface areas and/or greater total active air gap surface area than conventional motors having the same air gap diameter.
In addition to benefits of flux concentration obtainable with the configurations described above, recently introduced neodymium-iron-boron (NdFeB) magnetic materials can produce larger flux densities than other permanent magnetic materials previously used in brushless machines, thus increasing torque output capacity. The use of high density producing permanent magnets in motors which comprise a great number of poles presents a concern for ameliorating undesired effects that can be introduced by cogging torque. Cogging torque is produced by magnetic attraction between the rotor mounted permanent magnets and those stator poles that are not in a selectively magnetized state. This attraction tends to move the rotor magnet to an equilibrium position opposite a stator pole to minimize the reluctance therebetween. As the rotor is driven to rotate by energization of the stator, the magnitude and direction of the cogging torque produced by magnet interaction with non-energized electromagnet segments changes periodically to oppose and, alternately, to add to the torque produced by the energized stator segments. In the absence of compensation, cogging torque can change direction in an abrupt manner with the rotation of the rotor. If cogging torque is of significant magnitude, it becomes a rotational impediment, as well as a source of mechanical vibration that is detrimental to the objectives of precision speed control and smooth operation.
As an illustration of the development of cogging torque, a motor such as disclosed in the copending application Ser. No. 09/826,422, filed Apr. 5, 2001, is considered. The disclosure of that application has been incorporated herein. FIG. 1 is an exemplary view showing rotor and stator elements. Rotor member 20 is an annular ring structure having permanent magnets 21 spaced from each other and substantially evenly distributed along cylindrical back plate 25. The permanent magnets are rotor poles that alternate in magnetic polarity along the inner periphery of the annular ring The rotor surrounds a stator member 30, the rotor and stator members being separated by an annular radial air gap. Stator 30 comprises a plurality of electromagnet core segments of uniform construction that are evenly distributed along the air gap. Each core segment comprises a generally u-shaped magnetic structure 36 that forms two poles having surfaces 32 facing the air gap. The legs of the pole pairs are wound with windings 38, although the core segment may be constructed to accommodate a single winding formed on a portion linking the pole pair. Each stator electromagnet core structure is separate, and magnetically isolated, from adjacent stator core elements. The stator elements 36 are secured to a non-magnetically permeable support structure, thereby forming an annular ring configuration. This configuration eliminates emanation of stray transformer flux effects from adjacent stator pole groups. Appropriate stator support structure, which has not been illustrated herein so that the active motor elements are more clearly visible, can be seen in the aforementioned patent application.
FIG. 2 is diagram of a partial plan layout of a motor such as illustrated in FIG. 1, with stator poles shown in relation to rotor permanent magnets 21. Stator core elements 36 each comprise a pair of poles having base portions 31 and pole shoe portions 32. The poles are integrally linked to each other by linking portion 33. Energization windings, not shown, for each pole pair may be formed in well-known manner on the pole base portions or on the linking portion.
FIG. 3 is a partial plan layout of two adjacent stator core elements 36, with pole faces 32 denominated A-D, in relation to the rotor magnets, denominated 0-5, during motor operation. The positions of the rotor magnets are depicted at (A)-(C) for three instants of time (t1-t3) during a period in which the rotor has moved from left to right At time t1, the winding for the A-B stator pole pair is energized with current flowing in a direction to form a strong south pole at A and a strong north pole at B. The winding for the C-D stator pole pair is not energized. The position of the rotor is shown at (A). North magnet 1 and south magnet 2 overlap stator pole A. South magnet 2 and north magnet 3 overlap stator pole B. At this time magnet 3 is approaching an overlapping position with pole C. South magnet 4 is in substantial alignment with pole C and north magnet 5 is in substantial alignment with pole D. At this time motoring torque is produced by the force of attraction between south pole A and north pole magnet 1, the force of attraction between north pole B and south pole magnet 2, and the force of repulsion between north pole B and north pole magnet 3. Poles C and D have respective weak north and south magnetization caused by the attraction of magnets 4 and 5. This attraction, which seeks to maintain minimum reluctance is in opposition to motor driving torque.
At time t2, the rotor has moved to the position shown at (B). The energization of the pole pair A-B windings has been commutated off. Windings of the C-D pole pair are not energized. Magnets 1 and 2 are substantially in alignment with poles A and B respectively. North magnet 3 and south magnet 4 overlap pole C. South magnet 4 and north magnet 5 overlap pole D. Poles A and B have weak south and north magnetization respectively. The stator poles C and D are influenced by both north and south rotor magnets. Pole C is in a flux path between north pole magnet 3 and south pole magnet 4. Pole D is in a flux path between south pole magnet 4 and north magnet pole 5. A cogging torque thus has developed that opposes the motor driving torque and changes in magnitude as the rotor magnets move from direct alignment with the non-energized stator poles to partial alignment
At time t3, the rotor has moved to the position shown at (C). Energization of the A-B pole pair windings has been reversed, causing a strong north pole at pole A and a strong south pole at B. Windings of the C-D pole pair are not energized. North magnet 1 and south magnet 2 overlap stator pole B. South magnet 0 and north magnet 1 overlap stator pole A. At this time south magnet 2 is approaching an overlapping position with pole C. North magnet 3 is in substantial alignment with pole C and south magnet 4 is in substantial alignment with pole D.
As described above, the opposing cogging torque effects motoring torque in a manner that varies with respect to rotational angular position as the rotation proceeds. The cogging torque is most pronounced at transitional points when a rotor magnet is about to face a stator pole across the air gap. An abrupt change in the cogging torque takes place as the leading edge of the generally rectangular surface of a permanent magnet approaches the parallel edge of the rectangular stator pole. Use of high energy density permanent magnet materials such as neodymium-iron-boron (NdFeB) magnetic materials, which impart large flux densities at the air gap in the vicinities of the rotor permanent magnets, heightens this effect to the extent that undesirable vibration can become noticeable. Motors having a large number of stator poles and rotor poles, such as the axially aligned rows of stator poles and rotor magnets, can produce even greater cogging torque effects. In the same manner, cogging torque is produced to a varying extent in motors having unitary stator cores.
A variety of techniques have been utilized to minimize the effects of cogging torque. Such techniques attempt to reduce the rate of reluctance change with respect to rotor position, reduce the magnetic flux in the machine, or shift poles in a unitary stator core such that the cogging torque produced by the individual poles tend to cancel one another. Electronic methods can be used to control the intensity of the electromagnetic interaction that takes place between permanent magnet and electromagnet surfaces. Such methods have disadvantages in that they involve complex control algorithms that are implemented simultaneously with motor control algorithms and tend to reduce the overall performance of the motor. Reduction of magnetic flux diminishes advantages obtained from the newer permanent magnet materials and the flux concentration techniques of the above-identified copending applications. Shifting the location of poles in a conventional unitary stator core structure poses limitations on the size, positions and number of poles, which can prevent an arrangement that provides optimal operation.
Other approaches involve modifying the construction of the machine by changing the shape of the stator poles. Prior art stator poles conventionally made of stacked laminations are not readily amenable to modification. Available lamination machining processes are limited in the ability to reshape conventional patterns, especially three-dimensionally. A substantial range of modification of such laminated structures is too complex and costly to be feasible.
The need thus exists for effective cogging compensation in motors, particularly those having high flux density magnitudes and concentrations, and do not detract from the efficient operation and control capability of the motors while providing practicability of cost and application.
Copending application Ser. No. 10/160,257 addresses this need by shaping stator pole surfaces or rotor magnet surfaces so that the stator pole surface geometric configuration and the rotor magnet surface geometric configuration are skewed with respect to each other. The effect of the skewing arrangement is to dampen the rate of change of cogging torque that is produced by the interaction between a rotor magnet and a pole of a non-energized stator electromagnet as the permanent magnet traverses its rotational path. The ability to selectively shape stator poles is made feasible through the use of core materials such as a soft magnetically permeable medium that is amenable to formation of a variety of particularized shapes. For example, core material may be manufactured from soft magnet grades of Fe, SiFe, SiFeCo, SiFeP powder material, each of which has a unique power loss, permeability and saturation level. These materials can be formed initially in any desired three dimensional configuration, thus avoiding the prospect of machining an already formed hard lamination material.
Copending application Ser. No. 10/160,254, filed Jun. 4, 2002, addresses the need described above by offsetting the effects of cogging torque produced in a plurality of axially spaced sets of rotor and stator elements. Poles of each separate axially disposed stator core are shifted or offset with respect to each other in the axial direction to cancel the effects of cogging torque without limiting the positional relationships among the stator poles in the circumferential direction. Alternatively, rotor permanent magnets, which are arrayed in the circumferential and axial directions are offset with respect to each other in the axial direction to cancel the effects of cogging torque without limiting the total number of permanent magnets or their positions in the circumferential direction.
Minimization of torque ripple and cogging torque effects without detrimentally affecting torque output capability continues to be an important objective.
The present invention fulfills this need, at least in part, by selective variation of the radial distance between an interfacing pair of rotor permanent magnet and stator pole along the circumferential length of the pair. The motor rotor has a plurality of permanent magnets, of substantially the same length in the circumferential direction, distributed circumferentially about the axis of rotation. A plurality of stator poles are distributed about the air gap, the poles all being of substantially the same length in the circumferential direction as the length of the magnets. The effects of cogging torque on the overall torque signature can be controlled by setting an appropriate air gap variation configuration, designated herein an air gap profile for ease of explanation, for an interfacing stator pole and rotor magnet. The air gap profile is the variation of the radial distance across the air gap between a stator pole shoe and a facing rotor magnet from one end of the pair to the other.
An appropriate air gap profile is dependent upon desired motor operating conditions and motor parameters, such as the number of stator poles and rotor magnets, winding energization sequences and other expected conditions. The profile can be obtained by varying the radial distance from the axis of rotation of either the surface of the rotor magnet or the surface of the stator pole. Either the rotor magnet surface or the stator pole surface may be at a constant radial distance from the axis, while the other surface is of a variable configuration. Alternatively, both the rotor magnet and pole shoe distances may be variable. In the preferred embodiments, the air gap profiles are the same for all interfacing rotor magnet and stator pole combinations. That is, all stator poles are of the same configuration and all rotor magnets are of the same configuration.
One such air gap profile within contemplation of the invention provides a substantially uniform decrease in the radial distance between the interfacing rotor and stator pair from a first end of the pair to the second end. If the rotor permanent magnets are of relatively constant thickness, the stator pole surfaces are sloped relative to the surfaces of the rotor magnets. Alternatively, the permanent magnets may decrease in radial thickness from end to end.
In another air gap profile, the rotor magnet surfaces may be each at a constant radial distance from the axis while stator pole shoes are of variable radial thickness with concave surfaces facing the air gap. The degree of concavity can be set in accordance with whether the rotor surrounds the stator or the stator surrounds the rotor. As a variation, the permanent magnets may be of variable radial thickness with concave surfaces facing the air gap of selected degree of concavity.
The above described pole structures can be provided, with advantageous results, in a stator arrangement having a plurality of separated, ferromagnetically isolated electromagnet core segments. Each segment may be formed of a pole pair, such as shown in FIG. 1. The stator is a single annular ring encompassing a single pole in the axial direction and a plurality of pole pairs in the circumferential direction. In other arrangements, multiple rings of stator poles are axially spaced, formed by a plurality of separated, ferromagnetically isolated, electromagnet core segments. Each of the core segments comprises a plurality of poles integrally joined by one or more linking portions extending generally in the direction of the axis of rotation. The stator thus forms a plurality of poles in the axial direction with a single pole of each segment distributed in the circumferential direction in each ring. In the latter arrangements, the rotor is formed of axially spaced rings of separated magnets disposed circumferentially along the air gap, the number of rotor rings being equal to the number of stator poles in a stator core segment.